414 



Ont =1.5 



cavitation could be established and no bubble 

 cavitation occurred near midchord at inner radii. 

 Both phenomena are suspected to be caused by a lack 

 of nuclei. So electrolysis was applied, as will be 

 discussed in the next section. 



6. VARIATION OF NUCLEI CONTENT BY ELECTROLYSIS 



Some measurements in the NSMB Depressurized Towing 

 Tank with the scattered light method indicated 

 that the nuclei content of this tank was nearly 

 independent of the pressure. The density of small 

 nuclei (17 ym) was 1.2xlo'^ m~3 and that of the 

 largest available nuclei (45 ym) was 1.2x10^ m . 

 [This corresponds with nuclei number densities, as 

 defined by Gates (1977) of 9xloll and 2.4x10^° 

 respectively]. A description of the measuring 

 technique which was used is given by Keller (1974) . 

 A comparison with similar measurements in the NSMB 

 large cavitation tunnel [Arndt and Keller (1976) ] 

 shows that the nuclei content is lower than that 

 in the cavitation tunnel at the lowest air content 

 by a factor of about 5 . The nuclei content in the 

 cavitation tunnel was very much dependent on the 

 total air content of the water, showing variations 

 of a factor of 10 between high (12.5 ppm) and low 

 (6.3 ppm) air content. This dependency was absent 



-'NT 



= 0.5 



Re. 



1.29 xlO"- 



FIGURE 20. Turbulent streaks inside the critical 

 radius at higher Reynolds numbers. Propeller C at 

 30% slip. 



at inner radii, where the minimum pressure exists 

 near midchord. At propeller A at r/R=0.6 the cavita- 

 tion index is between 0.13 and 0.20, at aj^r|,=0.5, 

 while the minimum pressure coefficient is 0.25. 

 As can be seen in Figure 22 no bubble cavitation 

 occurred. The cavitating spot at midchord is a 

 dent in the propeller surface and illustrates the 

 low local pressure. Similar observations were made 

 with propeller C at ajjrp=0.5. No threshold for sheet 



PAINT OBSERVATION 



Re^: 0.73x10 



,6 



CAVITATION OBSERVATION 

 ReM = 0.73x10® 



CAVITATION OBSERVATION 

 Re^ = 1.56x10^ 

 Gk,t = 2.09 



FIGURE 21. Effect of Reynolds number on. cavitation 

 inception outside the critical radius. Propeller A 

 at 40% slip. 



